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1,780 Posts
Discussion Starter · #1 ·
Dear 4C forum members,

I think we've done it...finally. The idea of adjustable ARB's started in 2018 and I started experimenting in 2019. It looked very promising with first prototypes, but then the challenges and issues just kept coming at us. The more we were trying, the less it seemed that it will be possible to achieve what we wanted - a lightweight and efficient ARB's.

For those not following our car development thread where we weekly post updates, let me tour you through a quick timeline how it all went. In 2019 we designed first protoype ARB's. Hollowed tube and blade type levers, CNC machined out of T6-6061 alu. The front ARB was piece of cake, so we quickly moved to the rear ARB and that's where the challenges really started. Rear ARB is bent in 3 planes and very difficult to fabricate. We couldn't make ARB blade levers to fit OEM suspension, it was a tight fit on aftermarket coilovers and due to its complex shape, the part was heavy - too heavy for my liking. So we ditched the blade lever type idea and started experimenting with CNC bending of ARB tubes. Front, again easy-peasy, rear, impossible. We wanted to keep hollowed tubing to save the weight, but large diameter and that's where the challenge started. It was impossible to bend rear ARB shape in 3 planes in large diameter without cracking the hollowed tubes. We were using a spring steel tubing, but none of the CNC bending companies were up to the task. They all accepted the project at first and had no second thoughts about it even though we provided all the technical drawings etc. However, once they started bending the rear ARB, the CrMo tubing just kept cracking as the radius for the rear ARB are very tight, it's bent in 3 planes and due to larger diameter and tight clearance in 4C, there was no room for error. Alternative was softer tubing or solid bar, but I insisted on having hollowed bars, otherwise there is no point. It would be just another aftermarket ARB way too heavy for the 4C. No way! So we tried different material hollowed tubing, made the shape using CNC bending for front and rear ARB, but the performance just wasn't there. After a few sessions the ARB's fatigued and dropped the stiffness noticeably. So were were again back at the start - finding a solution to use a spring steel tubing that we could bend and would perform as required. Now, I'm not gonna post every detail how many different materials, diameters and bending companies we tried to finally tackle this challenge, but at the end, I ran out of companies that wanted to further cooperate, even though we well paid all the work and samples. They just didn't want to bother anymore and I still have more than 15 sets of failed ARB's each with a problem and at least twice as much of different tubes and unfinished prototypes. I'm afraid to make a calculation of costs because it hurts. I was so close to giving up, but driving the car with some of the final prototypes which performed flawless in terms of performance, just looked bad (wrinkled bending), kept me going. I really didn't want to give up. Then one day, on the far end side of our country, I found another CNC bending company that specializes in CNC bending and makes parts for MB. They are big, so I knew they won't be interested in making 10 sets of ARB's for our testing. So I sent them e-mail and made a glorious presentation of what we do and who we are. Much bigger than we actually are, because I didn't wanted to get turned down. They called me a few days later and we scheduled a meeting. I gave them the technical drawings, the sample materials and didn't even ask for a price because I didn't really have any choice - no other company wanted to waste time with our project anymore. So in a few weeks they called and said that it is impossible to make a rear ARB bends without cracking in this material and sent me an invoice. That was the moment I really said to myself "Give it up, we can't do it, it's OK, you can't always make it". Being so close but so far, I just couldn't sleep. I went through all the material specs, certificates etc., and realized that we were right there on the edge of what we wanted to do and what actually can be done with given material (bending radius, fatigue, strength, heat treatment, etc.). So next days I gave them a call again and said: "I'm sure it can be done, I have all the CrMo tubing supplier certificates, tech specs., etc. and it can be done. Now you either don't know how to do it or you don't want to. I'm not asking you to do it for free. I already did cover all the expenses of fails, so at least listen to me and please try it one more time with different CNC bending machine specs, please, don't tell me it can't be done." I'm not sure whether I sound so desperate or so nasty, but they did give it another try. I got called 3 weeks later and they said "It's done". My first thought was: "They must have changed the material and it's gonna be shit". I don't believe they've done it". They explained it to me, that they've set-up the CNC bending machine parameters to a specs they haven't done before (speed, pushing force, clamping force, heat, etc.) and that they've nailed it. A bit skeptical and anxious I was waiting for the tubing to arrive. In a few days I got delivered a 10 pairs of front and rear ARB tubing. The bends looked immaculate, so I grabbed a pair of tubes and ran to a fellow company to make a material analyze and it truly was the material we insisted on - a hollowed 4140 CrMo in 25mm and 28mm diameter and it was bent to perfection. I haven't been so lucky for a long time. I've been running a "wrinkled" version of this 4140 assembly since 2021 and now in 2022 we finally made it to our standard. So, the long story short, it took us more than 3 years, to finally fabricate what I had in mind - lightest aftermarket ARB on the market, without compromises. It was painful and costful journey, but I'm happy now. My mind is set at peace and I can finally sleep tight now.

Forgive me for bothering you with such a story, but I had to let it out. It was one of the most exhausting projects I've encountered and I'm so happy to share the final project here with you. I'm honored to invite you to have a look at the pre-final assembly.

Idea and development since 2019:

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1,281 Posts
What a path, Rudi! I admire your strength, resilience and perseverance. What a great product this has created! Congratulations 👏🏻.

Out of ‘curiosity’: can ARBs be used before mounting other arms or suspension? Are there gains to be made? Could you suggest your vision in the follow-up of upgrades.

Greetings. Big day for Slovenia 🇸🇮 I read in the news.

Premium Member
3,864 Posts
Congrats, Rudi. This seems like it was a hell of a project. It must be extremely satisfying to finally get it right. I know we all thank you for your diligence and efforts in making these improvements to our cars.

1,780 Posts
Discussion Starter · #11 ·
Specs below...

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  • Stiffness marked with (*estimated) were calculated based on OEM design and specific diameter of the given ARB. Position of the adjustment holes, materials and ARB end links position all affect the actual stiffness, but were not taken into account, so the values are estimates.
  • None of the stiffness values don't take into account the actual used ARB material which is crucial. Using regular iron bar or mild steel bar will have basically split quality of the mechanical properties compared to high tensile 4140, which is then even further post-processed (shot peening).
  • We did test ARB's made of SS and regular iron bar and we did notice the drop in the stiffness at later stages of testing, because the material simply fatigues and looses it's characteristics. Imagine bending back and forth a pen clip on the pen cap. At some point it fails. Sometimes it just cracks, other times, if material is "soft" it just keeps getting softer and softer, to the point it has virtually no elasticity in itself anymore and won't return to it's original position anymore. Same happens with ARB's made of improper materials. The torsional forces acting on the ARB will weaken the material to the point of ARB either cracking or simply not doing its function anymore.

1,780 Posts
Discussion Starter · #13 · (Edited)
In short, yes.

In long...below.

Suspension, steering and chassis setup is very complex. ARB's, springs, damping, alignment, they all affect the handling and are tightly related to each other. Changing one spec, affects others. It is impossible to say that one setup is the best on all aspects and drive conditions (weather, track, surface...). It might be an overall good setup, that works decently on a street, yet is very capable on a track, but it's not perfect. It's a compromise between these two. Track cars are too stiff on a road and street driven cars are too soft on the track. Damping (rebound mostly) and ARB's allow you too quickly fix that to a certain level, so ARB's are a good tuning tool and are used also in F1 and other top level motorsports, however they do have limitations and are not to be considered as the ultimate upgrade to perfect suspension setup. They should be considered as a part of suspension tuning. Personally, in my previous cars, I always invested in ARB's first as this was the easiest and cheapest fix to get less body roll.

ARB's are a very effective tool when it comes to tuning the suspension, however there are pros and cons. By working smart you the only penalty is added weight, which in our case is same as with OEM setup, yet you get adjustability and increased roll stiffness. Automakers prefer to add ARB's to rather soft spring setup, over stiff springs and no ARB's, because this way they prevent the car from excessively body rolling, yet maintain the ride comfort. 4C is such case. OEM springs have surprisingly low spring rates so the car feels rather comortable on the street. But the body roll is quite bad as the ARB's cannot entirely compensate for low spring rates. ARB's can be used to stiffen up the car in corners and prevent excessive body roll, especially on stock setup, but they won't solve the dive and squat issue.

Adjustable ARB's are far superior to non adjustable swaybars, because as the name says it, - they can be adjusted, while non adjustable, are only on/off, which rules out the chance of adjusting understeer / oversteer characteristics. They are the best way too quickly change the understeer/oversteer characteristics of the car without changing the springs. Let's say you go to a fast open track, with long sweepers where you definitely don't want a tail happy 4C, so you put the rear swaybar from stiff to soft and you get much more planted rear end, with entire car more prone to understeer, which in this case, on this track, won't be an issue as understeer is a slow corners issue. Now, you're running a decent lap times and the rain is coming for the final run. This same setup will be much too stiff in wet to get any decent lap times out of it. The grip will be severly reduced, therefore the alignment (negative camber) will be far too agressive to utilize entire tire patch as the car will lean far less now and you'll end up with a totally incompetitive setup. Again, swaybars are your savior. Soften or even disconnect them to allow the car to body roll more, to get the tire patch back to the ground and generate more grip. Next week, you go for an auto X with slow and tight corners, where this same setup will understeer heavily. Reconnect the swaybars, set the front to soft and rear to stiff and the car will nicely rotate through the tight corners with this prone to oversteer setup. None of this adjustments are possible without adjustable swaybars, unless you change the springs, which is not really doable withotu entire motorsport team covering you.

Ditching the ARB'S will save the weigh, but will require you to run much stiffer springs and more aggressive alignment to compensate for body roll and you will be left without any adjustment for understeer/oversteer. Car will body roll noticeably more, unless the spring rates will be very high, but that could cause the car skipping over road imperfections, as it will be generally too stiff. To compensate for body roll you need very aggressive negative cambers, but this then decreases braking performance and straight line stability. In 4C's case, it is the design of the swaybars, that renders them not too effective. They have very long leverages (rear) are hollowed and small in diameter, so in OEM format they don't contribute much to the body roll resistance. However, going too stiff with ARB's will cause other issues. The car won't roll at all and it will be very hard to "read", will provide poor feedback as all the weight transfers will happen immediately. I've tried such a setup too and the car was almost undrivable. There was no warning to what's going to happen (understeer / oversteer), it just snapped and the grip just wasn't there. There must be some level of body roll, nose dive and squat for optimal performance. This will provide great feedback and render the car "driveable". Too much of ARB stiffness will also greatly contribute to the suspension binding which is mostly noticeable on rough surface and not much on the track. Some car run beam axle at the rear (100% binding), yet they are fast on the track, because the surface is smooth and suspension travel is only few cm up and down, so suspension binding due to ARB'S is not really a problem in track cars. On other hand, in extreme case - offroad, you don't want ARB's at all, as you want to allow the tires to work absolutely independently to maintain the contact with the surface.

So what's my current setup and what was the chronology:
  • 1st setup - Standard OEM suspension, 22m OEM front ARB
    • too much body roll, squat and dive
  • 2nd setup - Nitron Coilovers 52N/mm F / 70 N/mm R, 22m OEM front ARB
    • less body roll, less squat, less dive, but still worn out edges on -2deg of camber, tendency to slightly oversteer on the limit
  • 3rd setup - Nitron Coilovers 52N/mm F / 70 N/mm R, 22m OEM front ARB
    • less body roll, less squat, less dive, but still worn out edges on -3deg of camber, tendency to slightly oversteer on the limit
  • 4th setup - Nitron Coilovers 65N/mm F / 80 N/mm R, 22m OEM front ARB
    • less body roll, OK squat, OK dive, but still worn out edges on -3deg of camber, tendency to slightly oversteer on the limit
  • 5th setup - Nitron Coilovers 65N/mm F / 80 N/mm R, 28mm GMS ARB front and 30mm GMS ARB rear
    • not enough body roll, OK squat, OK dive, poor handling - hard to read the car on the limit, lack of grip, poor turn in, general understeer
  • 6th setup - Nitron Coilovers 65N/mm F / 80 N/mm R, 25mm GMS ARB front and 28mm GMS ARB rear
    • OK body roll, OK squat, OK dive, even tire wear on -3deg of camber, neutral oversteer / understeer balance
  • 7th setup - Nitron Coilovers 65N/mm F / 80 N/mm R, 25mm GMS ARB front and 28mm GMS ARB rear, GMS rear wing
    • OK body roll, OK squat, OK dive, even tire wear on -2.5 deg of camber, slightly prone to understeer, fastest laptimes
  • 8th setup - Nitron Coilovers 65N/mm F / 90 N/mm R, 25mm GMS ARB front and 28mm GMS ARB rear, rear wing
    • Not properly driven yet / time set. We expect OK body roll, OK squat, OK dive, even tire wear on -2.5deg of camber and neutral understeer / oversteer balance which is achieved by stiffer rear spring rates to compensate for added rear downforce.
These were only setups that either ARB's alignment or suspension were changed. In between these, we also exprimented with alignment, damping settings, tires, control arms geometry etc., but that's not relevant at this stage.

Regarding the spring rates, for mostly street use, I would say that 52N/mm F and 70 N/mm rear with adjustable ARB's is the sweetspot. Not noticeably stiffer (over OEM standard suspension), on the street with ARB's and damping set to soft, yet decently cable on the track with ARB's set to hard and coilover's damping cranked up. For dedicated track use, 65N/mm F and 80 N/mm R, cranked up damping and ARB's set to hard it is. For cars with significant aero, the spring rates should be uprated accordingly.

1,780 Posts
Discussion Starter · #16 · (Edited)
So, here we are, finally.

Today, 20.05.2022 at 24:00 CET, we are officially launching this new upgrade - GMS SLR stabilizers / ARB's / swaybars, anti-roll bars Unfortunately, due to other priorities we have to attend lately (workshop construction), we are very limited with time, so we couldn't fabricate a decent batch, so at this point, we'll only be able release a few sets. We'll have more done in a month or so, so thank you all for your patience. I hope you find our engineering up to your taste, we did our best to bring this to a realization.

Here is the link: ALFA ROMEO - 4C - SLR Stabilizers / Anti-Roll Bars | GMS

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  • GMS assembly 25mm and 28mm (w/ ARB links) 6211g
  • OEM assembly 21mm and 22mm rear (w/ ARB links) 6605g
  • Random aftermarket assembly I 22mm and 24mm (w/ ARB links) 9180g
  • Random aftermarket assembly II 24mm and 27mm (w/ ARB links) 12760g

1,780 Posts
Discussion Starter · #19 ·
It's hard for me to judge it for you how much of a difference it makes. In my case, on a track, enough to allow me to run -2°30' of camber instead of -3°00 on already relative stiff setup (65N/mm front springs and 80 N/mm rear springs). On a street the increased ARB stiffness is felt even more, because it's hard to generate same amount of grip and consequentially body roll, on a street as on the track, so even slight increase in body roll stiffness makes a noticeable difference.

If I were to mod the 4C with planed budget, here is how I would do it:
  1. Camber and caster plates + rear uniballs / rear control arms
  2. Coilovers
  3. ARB's
  4. Front uniballs / front control arms
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